Mucus droplets. Coloured scanning electron micrograph (SEM) of droplets (red) of mucus in the trachea (windpipe). The droplets are seen on epithelial cells. Mucus is a substance secreted by many interior epithelial surfaces, such as the lining of the lungs, the nasal passages, and the digestive and reproductive systems. Mucus consists of various substances depending on location, but generally has a lubricating function. The trachea is the tube that leads from the mouth to the lungs, and the mucus traps airborne pathogens and particles.